Oil & Natural Gas Corporation Ltd, Vadodara

Email: joyelsy@hotmail.com

10th Biennial International Conference & Exposition

P 157

Exploration Potential of Syn-rift Olpad Play in the western rising flank of

Warosan low, Mehsana Tectonic Block, North Cambay Basin.

Varughese C. M.*, Das N.C., Arya O. P., Kumar Anil & Singh A. K.

Summary

Mehsana area of North Cambay Basin has got well established oil fields which are producing for last four decades from

multiple plays at different stratigrapic levels from Older Cambay shale to Kalol. The basin is in a very mature state of

exploration and of late it is observed that though the success rate is good but sizes of the discoveries are small. Hence there

is need to shift focus of exploration to deeper plays below Older Cambay Shale (OCS) and in this respect exploration of Syn-

rift Olpad Formation is important. The encouraging result from recently drilled exploratory well in the eastern basin margin

in the North-Mansa area, which is producing oil from Olpad on self at a sustained rate of 190 barrels/day with no drop in

reservoir pressure, has rejuvenated the pursuit for hydrocarbons in the synrift sediments in north Cambay Basin. The present

study is an effort assimilate this success and to look for similar setup in other parts of the Mehsana area. The eastern margin

success of Olpad has occurred in a set up where the Cambay shale, which is the proven Source rock, is in juxtaposition with

Olpad Formation. The authors are of the view that similar setup exists in the western rising flank of Warosan low also.

Keywords: North Cambay Basin

Introduction

Cambay Basin is a narrow elongated (NNW - SSE)

extensional basin located on the western margin of Indian

plate (Fig. 1). Precambrian rocks of Aravalli System are

exposed in the northeast. Deccan Trap of late Cretaceous

to early Tertiary age with underlying Mesozoic strata is

exposed on the eastern and western flanks of the basin.

This Cenozoic extensional basin is a typical “intra cratonic

aborted rift” which evolved due to rifting along

Dharwarian orogenic trend during the northward migration

of the Indian plate after its break up from Gondwanaland

in Late Mesozoic and the Basin came into existence during

Early Cretaceous. The rift-drift transition phase of Indian

plate witnessed volcanic events in the western India during

which huge thickness of traps are deposited in Cambay

Basin.

The extensional architecture of the basin is defined by two

types of faults viz., 'listric normal faults', striking N-S to

NNW-SSE and 'transfer faults', trending ENE-WSW to

NE-SW. The listric faults mostly run sub parallel to the rift

- border faults. Transfer faults frequently offset the listric

faults. During Late Miocene, few areas in the basin

experienced inversion tectonics related to Himalayan

Orogeny. Thus basin architecture is defined by an

enechelon arrangement of asymmetric half grabens

Fig.1 Geological map showing Cambay Basin

2

bordered by listric normal faults oblique to the rift axis and

are separated by transfer fault zones/accommodation zones

and basement highs. A thick sequence of sedimentary

rocks ranging in the age from Paleocene to Recent overlies

the Deccan Trap, which is considered as technical

basement in Cambay Basin. A total of about 6 km thick

sedimentary sequence is expected in the deepest part of the

Cambay Basin. A complex network of these faults

compartmentalizes the basin into distinct

tectonosedimentary blocks bordered by major transfer

faults (Fig 2). Based on major basement faults the Cambay

Basin is subdivided into five major tectonic blocks, which

Fig.2 Tectonic map of Cambay Basin.

have been named from south to north Viz. (I) Narmada-

Tapti Block, (II) Jambusar-Broach Block (III) Tarapur-

Cambay Block (IV) Mehsana-Ahmedabad Block and (V),

SanchorPatan Block. The first two blocks informally

comprise South Cambay Basin and last three blocks are

North Cambay Basin separated by Mahi River. The present

study is from northern part of Mehsana- Ahmedabad

Block. Exploration in North Cambay Basin hitherto was

mostly confined to lower Eocene synrift to post rift plays

pertaining to Older Cambay shale, Younger Cambay shale

(Kadi & Kalol formations). The exploratory efforts so far

were based on structural elements and have met with

considerable success in finding hydrocarbons from

multiple stratigrahic levels (Fig.3). The major oil fields

(Fig.4) in this area are Lanwa-Santhal-Balol producing

exclusively from Kalol Formation. Jotana, North-Kadi,

Sobhasan, Linch and Nandasan produce from multiple

plays ranging from Older Cambay shale to Kadi and Kalol

Formations. Exploration has reached a mature stage and

at present it is focused on subtle traps and small amplitude

entrapment situations in the areas bordering established

fields.

Huge thicknesses of Paleocene synrift sediments (Olpads)

in Mehsana area of Cambay Basin has escaped focused

Fig.3 Generalized Stratigraphy of Cambay Basin.

exploration over the years due to the successes from

younger plays. Though Small oil pools in Olpad are

already established on Khambel-Kamboi and West Patan

area to the west of Mehsana Horst, success was so far

elusive in the eastern depression and eastern margin.

Fig.4 TWT Relief Map on horizon within Younger Cambay

Shale.

3

Reservoir characteristics & Play types

In the early stages of rift the primodial lineaments

reactivated; subsequent rift & drift lead to creation of

undulating Basin floors with highs and Lows providing

depocentres for huge thickness of sediments i.e, Olpads

derived from erosion of Deccan traps.

The commom lithofacies of Olpad comprises trap

conglomerate with pebbles of varying sizes (Fig.5), trap

wacke, trap wash, siltstone & claystones. Trap

conglomerates and trap wacke differ in size of pebbles and

trap fragments generally embedded in clayey matrix. The

finer volcanic particles known as trap-wash (Fig.6)

comprise clay and siltstones. Claystones are often

laminated, varying in colour from dark grey, blackish grey

to red. Thinly bedded crypto crystalline dirty white layers

are also reported from Olpad Formation suggestive of

deposition under low energy conditions. Bioclastic

trapwacke comprising sub rounded basalt clasts along with

shell fragments embedded in chloritic matrix (fig.7)

suggestive of fluvial/lacustrine condition are reported in

wells in Charada area in eastern margin.

Olpad play can be broadly divided into two major

categories based on their occurrences and genetics; A.

Sediments on Basin margin, structural highs and their

flanks in narrow half grabens. B. Rift fills sediments in

depressions. Olpads on Basin margin, on Horsts blocks and

its flanks can be explained by insitu weathering

and leaching of traps and transport of these huge

volumes by alluvial fans towards nearest depocentres

/depressions.

The resultant deposits exhibit the occurrence trap

conglomerates gritty sands at proximal part of alluvial

fans, trapwacke on the crest and immediate slopes and

finally grading into trapwash comprising clays, siltstone

facies further downward in depression. Major depressions

both west and east of Mehsana Horst has accommodated

huge thickness of Olapd sediments as they became more

pronounced with rifting till major marine transgression

during which Older Cambay shale was deposited in the

entire Basin. In the depressions however these Olpad

sediments might have been affected by axial drainage

resulting in development of good reservoir facies at places.

Fig.5 FMI showing Trap conglomerate with gravel/pebbles of

different sizes in Olpad Formation in Charada area.

Fig.6 FMI showing interbedding of Trapwash & Trapwacke in

Olpad Formation in Charada area.

4

Fig.7 Photomicrograph of SWC in a well in Charada with Basalt

clasts & shell fragments (Bioclastic trapwacke) embedded in a

chloritic matrix.

Discussion

So far, success in synrift Olpad sediments was limited to

west of Mehsana Horst i.e, in Khambel-Kamboi high and

West Patan area. Two wells in Khambel-Kamboi and one

well in West Patan (Fig.8) have produced oil in

commercial quantity.

Fig.8 Seismic section through Well WP-A which produced oil

from Olpad.

Many more structures were drilled in the similar horst-

graben set up in the areas west of Mehsana Horst, the

results were not encouraging. The possible reasons for this

may be insufficient quantum of hydrocarbon generation

due to the fact that sedimentary thickness is not much in

these grabens and shallow burial depth of source rock

facies. So far exploration activity for Olpads in east of

Mehsana Horst is sparse and success is limited to eastern

margin. In the recent past thick oil zones in Olpad were

established at shallow depths in three wells B, C &D

(Fig.4) in Charada area in the eastern margin of the basin.

The wells drilled Charada area have encountred

conglomeratic Olpad overlying the Trap (Fig. 9). Due to

Fig.9 Logs illustrating Olpad/Trap contact in Well-B in Charada

area in eastern margin.

highly viscous nature of the oil, these wells are yet to be

put under production, awaiting suitable and reliable

technology for exploitation.

The latest success from Olpad has come from a well-A in

North-Mansa area (Fig.4 & 10) again in the eastern margin

Fig.10 E-W Seismic section through Well-A, Producer

from Olpad.

which is producing oil on self at a sustained rate of 190

barrels/day with no drop in reservoir pressure, indicating

very good lateral extent of reservoir. The log motif of the

pay zone is shown in fig.11. This success has rejuvenated

the pursuit for hydrocarbons in syn-rift sediments in the

east of Mehsana Horst. The present study is an effort to

5

assimilate this success and to look for similar setup in

western rising flank of Warosan Low. We can understand

from the regional relief map (Fig.4) of the reflector within

younger cambay shale (Mandhali) that Warosan Low is

actually the northern extension of Nardipur Low. The

rising flanks of this composite Warosan-Nardipur Low is

important because of the fact that this depression has got

huge thickness of cambay shale, which is the established

source rock in the north Cambay Basin and we have major oil fields producing from shallower Formations. Unlike the

areas falling in the west of Mehsana Horst, this low might

have generated enoromous amount of hydrocarbon to

charge the reservoirs at multiple stratigraphic plays in the

east of Mehsana Horst area. Jotana Fault (Fig 4 & 12) with

significant throw is a prominent listric Fault east of

Mehasan Horst.

Fig.11 Composite log motif & processed log of Olpad pay zone

of a Well-A in North Mansa area.

Fig.12 Coherency slice at 1100 ms clearly brings out

Jotana fault.

Thickness of sediment layers/Formations gradually

increases eastward towards Warosan depression. At all

stratigraphic level, the sedimentary layers forms anticlinal

structure along the downthrown side of the Jotana fault due

to drag effect.

Fig.13 Regional E-W Seismic transect connecting Jotana field and Well-A of Mansa Field.

6

All along the length of this fault hydrocarbon

accumulations in OCS, Mandhali, Mehsana & Kalol have

been established and currently being exploited in Jotana

field, adjoining western rising flank of Warosan Low &

northern part of Linch field.

As discussed earlier, Mehsana Horst being the prominent

structural element, uplifted to considerable height during

rifting stage must have contributed enormous amount of

trap derived sediments/Olpads during upheavals to the

west as well into Warosan depression in the east. Jotana

Fault being the nearest major listric fault to the east of

Mehsana Horst, thick sequence of Olpads in the form of

trap conglomerate, trap wacke are likely to be encountered

in the western rising flank of Warosan Low and forming

entrapment against this major fault as well as in the up

thrown block bordering eastern flank of Mehsana Horst

where Olpad is in juxtaposition with Cambay shale which

is the established source rock. In the similar set up oil has

been encountered in well A which is evident in seismic

section (Fig. 10). It can also be seen from the seismic

transect (Fig.13) that similar setup do exists in the western

rising flank of Waosan low. It is well established fact that

Jotana main fault is providing the entrapment condition at

multiple younger stratigraphic levels. Till now the

exploration success along this fault was mainly from post

rift sequences. No well has penetrated Olpad Formation in

the immediate downthrown side of this major fault. Few

wells were drilled up to Olpad in the up thrown block of

this fault. Well E drilled in northern part of Linch field has

encountered good reservoir facies in Olpad. Well-F in

Santhal field in up thown side of Jotana fault has been

tested in Olpad and gave indications of of oil. Western

rising flank of Warosan depression where Olpad

Formation is in juxtaposition with Older Cambay shale is

likely to be the ideal locale for hydrocarbon accumulation.

Further eastward, towards basinal axis Olpad Formation is

likely to be dominated by trapwash and finer clastics being

away from source. Well-G was drilled to a depth of 5006m

in the above settings and encountered about 2000m of

Olpad sequence, however the reservoir facies were poor.

Introspection

In the available seismic data, resolution of deeper events

are very poor and are also masked by first orders multiples

of Kalol formation, hence impression of Jotana fault is not

very clear in the deeper section. Energy penetration beyond

Kadi formation is a problem in the area due to presence of

thick coal in Kalol and Kadi formations. Hence getting

proper seismic expression from Olpad formation which is

expected beyond a depth of about 2700 mts is a challenge

and recent efforts to acquire data by focusing this objective

was not very successful. Special reprocessing with focus

to enhance the deeper events may help in interpretation. It

can be concluded that the recent success in Olpad

Formation in the eastern margin, can be replicated in the

western rising flank of Nardipur-Warosan low also where

we find Cambay shale is in juxtaposition with Olpad. In

this part there is added advantage of the presence of major

Jotana fault which offers a proved entrapment situation.

Here the Olpad reservoirs is expected at a deeper depth in

comparison to eastern margin, hence quality of the oil

should be better.

Conclusion

Mosaic of en-echelon half grabens and grabens acted as

depocentres during early synrift stage in north Cambay

Basin. Synrift sediments of Olpad Formation comprising

trap conglomerate, trap wacke, trap wash, siltstone &

claystone derived from weathering and leaching of Deccan

Traps were deposited in fluvial and lacustrine

environment. Warosan-Nardipur Low, the prominent

depression in the Mehsana area received enormous

thickness of synrift sediments from west as well as from

eastern Basin margin. Significant hydrocarbon finds in

Olpad sediment in the eastern margin in Charada & Mansa

area led us to believe that areas in the western rising flank

of Warosan depression, in proximity to Mehsana horst are

ideal locale for exploration of Olpads

Acknowledements

The authors are thankful to the ONGC management for

granting permission to publish the paper and use the data

and the findings of various projects. We are also grateful

to Mr. S. K. Das ED-Basin Manager, Western Onshore

Basin, ONGC, Vadodara for the guidance and

encouragement. The views expressed in the paper are

solely of the authors and not necessarily of the organization

in which they are working.

References

Lithostratigraphy of Indian Petroliferous Basins, by

J.Pandey etal; ONGC,1993;unpublished report

Identification of Prefered Reservoir Development In Olpad

Formation For The Exploration Of Deeper Prospects

7

Broach Block, Cambay Basin by S. Mayor1, S.S. Sawkar2

etal, 5th Conference & Exposition on Petroleum

Geophysics, Hyderabad-2004, India PP 388-395

Insights on Olpad Formation: -Assessment of

Hydrocarbon Play Types in North Cambay Basin, India by

V.Pratap and S.C.Sonare, 6th Conference & Exposition on

Petroleum Geophysics, Kolkota-2006, India

Developing a suitable methodology for the evaluation of

payzones for effective reservoir characterization of Olpad

in Charada-Mansa area in Cambay Basin by B.K.Saikia,

Rajesh Chandra , CEWELL. ONGC, June, 2010;

unpublished report